A manhole provides access to an underground passage or confined area. The underground passage or confined area may contain public utility equipment, such as sewer lines, storm drains, electrical and telecommunication cables, etc. A manhole cover is a removable plate that forms a lid over the opening of a manhole. Manhole covers are used to prevent individuals and objects from falling into the manhole, as well as to prevent unauthorized access into the manhole.
Manhole covers are conventionally formed of cast iron, which makes them inexpensive, strong, and heavy, usually weighing more 100 pounds. The weight helps to keep them in place when traffic passes over them, and makes it difficult for unauthorized individuals to remove them. In addition to being constructed of cast iron, manhole covers may also be constructed of concrete, glass-reinforced plastic or other composite materials, and other materials, or any combination thereof.
Despite their significant weight, manhole covers can be dislodged in several ways. For example, an explosion within a manhole can cause a sudden pressure buildup that can dislodge the manhole cover. For example, gases (e.g., methane from sewage or natural gas from a leaking natural gas line) can become trapped in the space within the manhole, as well as within the passages or spaces connected to the manhole. The gas may be ignited, for example, due to a spark from a frayed power cable. Some explosions generate sufficient pressure to dislodge the manhole from its frame. However, higher-intensity explosions may propel the manhole cover up to 20 feet or more into the air. A heavy manhole cover flying through the air can be extremely dangerous or deadly. In addition to the human and property damage risk, individuals or objects may subsequently fall into the now-uncovered manhole.
Excessive rainfall and flooding can also dislodge manhole covers. For example, storm drain systems may become overfilled during periods of excessive rainfall. Water may flow through the storm drain systems and up through a manhole. Sufficient pressure from the water may dislodge manhole covers and “float” them away. The now-uncovered manhole can be obscured by dirty water, thereby providing a dangerous risk that an unwary victim may inadvertently fall into the manhole and into the storm drain system.